The invention relates to cutting elements for rotary drill bits for use in drilling or coring holes in subsurface formations.
In particular, the invention is applicable to cutting structures for rotary drill bits of the kind comprising a bit body having a shank for connection to the drill string and an inner passage for supplying drilling fluid to the face of the bit, the bit body carrying a plurality of cutting elements. Each cutting element, often in the form of a circular disc, is a two-layer or multi-layer element including a thin front cutting table formed of superhard material, usually polycrystalline diamond, having a front cutting face and a rear face bonded to the front surface of a substrate of less hard material, such as cemented tungsten carbide. The rear face of the substrate may be bonded to a carrier, which may also comprise cemented tungsten carbide, mounted on the bit body. In one common form of drill bit of this type, the carrier comprises a stud or post to which the cutting element is brazed, the stud or post being received and secured within a socket in the bit body.
It has been found that cutting elements of the above kind may sometimes be subject to failure by a phenomenon which may be referred to as "diametral splitting". That is to say, after the drill bit has been in use, perhaps for only a limited period, some circular cutting elements become cracked or split along a diameter extending away from the cutting edge of the element and in a plane which is generally perpendicular to the surface of the formation being cut. Such splitting may be accompanied by breakdown of the bond between the rear surface of the substrate of the cutting element and the surface of the carrier to one side of the split, with the result that one half of the cutting element breaks away from the carrier thus rendering the cutting element substantially ineffective. In another mode of failure the split has been found to extend through into the main body of the carrier itself so that a portion of the carrier becomes detached, taking with it one half of the cutting element.
The present invention sets out to provide a novel form of cutting element which is designed to limit the propagation of cracks in the substrate which originate at or in the vicinity of the cutting edge, and thus reduce the incidence of diametral splitting.